1. Field of the Invention
The present invention relates to a lens barrel of an optical system to image a subject, particularly to one with an improved impact tolerability, and an imaging device incorporating such a lens barrel.
2. Description of the Prior Art
A lens barrel of an imaging device as a digital camera or a silver-salt camera is contained in a camera body and that of a lens interchangeable camera as a single reflex camera is separable from a camera body. Both of the lens barrels may receive a large impact when a user accidentally drops the camera or hits another object.
There are various lens mount systems and one of them is a bayonet type. The bayonet mount type is to join relatively moving cylindrical elements by fitting the protrusions into their corresponding depressions and circumferential grooves. In the bayonet mount lens barrel, when receiving a large impact to a protrusion on a mechanically weak portion, the cylindrical element may be deformed, disconnecting the protrusions from the depressions and grooves, and making it impossible to perform the following shooting operation.
Specifically, the cylindrical elements are combined by fitting the protrusions on one male element into the depressions on the other female element and relatively moving the elements axially and relatively rotating them around the axis to insert the protrusion into the circumferential grooves. When provided with a clearance groove for the pin or a cam groove in a circumferentially large area, the strength of the male cylindrical element with the protrusions is insufficiently low, especially in a portion in which the clearance groove and the cam groove overlap with each other. Accordingly, the protrusion areas are susceptible to an impact and easily deformable. The protrusion are thus easily disconnected from the depressions.
In order to prevent the protrusions from disconnecting, the width of the protrusions can be increased. However, a small increase in the width exert little effect and a large increase exerts mechanical strengths but it is necessary to precisely process the front and back sides of the protrusion in terms of parallelism. Alternatively, the depth of the connection can be increased or the cylindrical element can be made of an aluminum alloy rod by cutwork instead of synthetic resin or else. This can make the cylindrical element less deformable against an impact.
However, there is a problem in increasing the depth of the bayonet connection that the lens barrel is enlarged in diameter since the radial thickness of the connecting elements needs to be greatly increased. Also, it takes a long time to manufacture the cylindrical element made of the aluminum alloy rod, which is nonproductive. Further, the weight thereof is heavier than that of a general element made from synthetic resin so that the top end portion of the lens barrel may be inclined downward when extended.
Japanese Patent Application Publication No. 2003-322786 (Reference 1), for example, discloses a bayonet mount lens barrel to increase strength against external force by increasing the rotary angle of the bayonet connection and prevent moving lens groups from collapsing by their own weight. It includes a first cylinder with protrusions and a second cylinder with slits matching the protrusions, to bayonet-connect the first and second cylinders by fitting the protrusions into the slits. The combinations of the protrusions and slits are differently shaped.
Reference 1 has a problem that the protrusions and slits in different shapes and different radial sizes cause an increase in the size of the lens barrel, specially, in radial direction.
Further, Japanese Patent Application Publication No. H09-211292 (Reference 2) discloses another example of a bayonet mount type lens barrel in which lens holders can be easily and accurately assembled. However, it does not address an improvement in impact tolerability.
Japanese Patent Application Publication No. 2009-169388 (Reference 3) discloses another example of a bayonet mount type lens barrel which includes a fixed cylinder, a first rotary cylinder rotatable around the axis relative to the fixed cylinder, a moving disposed inside the first rotary cylinder integrally moving with the first rotary cylinder along the axis and relatively rotating thereto around the axis, and a second rotary cylinder disposed inside the moving cylinder and having a through hole. The moving cylinder includes three or more protrusions on the outer circumference and the first rotary cylinder includes three or more matching slits and circumferential grooves so that the moving cylinder and the first rotary cylinder are coupled with each other by inserting the protrusions into the slits and grooves. Further, the second rotary cylinder includes pins, the moving cylinder includes a matching through hole, and at least two of the three or more protrusions are arranged so as not to overlap with the through hole on the axis. Thereby, upon receiving an impact, the connection of the protrusions and slits can be maintained irrespective of an extended position of the lens barrel.
However, the lens barrel in Reference 3 cannot exert sufficient tolerability to a large impact from a lens barrel's dropping or hitting another object.